Home experimental tasks
Exercise 1.
Take a long heavy book, tie it with a thin thread and
attach a rubber thread 20 cm long to the thread.
Put the book on the table and very slowly begin to pull on the end.
rubber thread. Try to measure the length of the stretched rubber thread in
the moment the book starts sliding.
Measure the length of the stretched thread with the book moving evenly.
Place two thin cylindrical pens under the book (or two
cylindrical pencil) and also pull the end of the thread. Measure length
stretched thread with uniform movement of the book on the rollers.
Compare the three results and draw conclusions.
Note. The next task is a variation of the previous one. It
also aimed at comparing static friction, sliding friction and friction
Task 2.
Place a hexagonal pencil on top of the book parallel to the spine.
Slowly lift the top edge of the book until the pencil starts
slide down. Slightly reduce the slope of the book and secure it in this
position by putting something under it. Now the pencil if its over
put on the book, will not move out. It is held in place by the force of friction.
static friction force. But it is worth weakening this force a little - and for this it is enough
flick your finger on the book - and the pencil will crawl down until it falls on
table. (The same experiment can be done, for example, with a pencil case, match
box, eraser, etc.)
Think about why it is easier to pull a nail out of the board if you rotate it
around the axis?
To move a thick book on the table with one finger, you need to attach
some effort. And if you put two round pencils under the book or
handles, which in this case will be roller bearings, the book is easy
will move from a weak push with the little finger.
Do experiments and make a comparison of the static friction force, the friction force
sliding and rolling friction forces.
Task 3.
In this experiment, two phenomena can be observed at once: inertia, experiments with
Take two eggs, one raw and one hard boiled. spin
both eggs on a large plate. You see that boiled egg behaves differently
than raw: it rotates much faster.
In a boiled egg, the white and yolk are rigidly bonded to their shell and
among themselves because are in a solid state. And when we spin
a raw egg, then at first we spin only the shell, only then, due to
friction, layer by layer, rotation is transferred to the protein and yolk. In this way,
liquid protein and yolk, by their friction between the layers, slow down the rotation
shells.
Note. Instead of raw and boiled eggs, you can spin two pans,
in one of which there is water, and in the other there is the same amount of cereals by volume.
Center of gravity. Exercise 1.
Take two faceted pencils and hold them in front of you parallel,
putting a line on them. Start bringing the pencils closer together. Rapprochement will
occur in alternating movements: then one pencil moves, then the other.
Even if you want to interfere with their movement, you will not succeed.
They will still move forward.
As soon as on one pencil the pressure became greater and the friction
the second pencil can now move under the ruler. But after some
time, the pressure over it becomes greater than over the first pencil, and
as friction increases, it stops. And now the first one can move
pencil. So, moving in turn, the pencils will meet in the very middle
ruler at its center of gravity. This can be easily verified by the divisions of the ruler.
This experiment can also be done with a stick, holding it on outstretched fingers.
As you move your fingers, you will notice that they, also moving alternately, will meet
under the very middle of the stick. True, this is only special case. Try
do the same with a regular broom, shovel or rake. You
you will see that the fingers will not meet in the middle of the stick. Try to explain
why it happens.
Task 2.
This is an old, very visual experience. Penknife (folding) you have,
probably a pencil too. Sharpen your pencil so it has a sharp end
and stick a half-open penknife a little above the end. Put
the point of a pencil forefinger. Find such a position
half-open knife on a pencil, in which the pencil will stand on
finger, slightly swaying.
Now the question is: where is the center of gravity of the pencil and pen
Task 3.
Determine the position of the center of gravity of a match with and without a head.
Place a matchbox on the table on its long narrow edge and
put a match without a head on the box. This match will serve as a support for
another match. Take a match with a head and balance it on a support so that
so that it lies horizontally. Mark the position of the center of gravity with a pen
matches with a head.
Scrape the head off the match and place the match on a support so that
the ink dot you marked was on the support. It's not for you now
succeed: the match will not lie horizontally, since the center of gravity of the match
moved. Determine the position of the new center of gravity and notice in
which side he moved. Mark with a pen the center of gravity of the match without
Bring a match with two dots to class.
Task 4.
Determine the position of the center of gravity of a flat figure.
Cut out a figure of arbitrary (any bizarre) shape from cardboard
and pierce several holes in different arbitrary places (better if
they will be located closer to the edges of the figure, this will increase the accuracy). Drive in
into a vertical wall or rack a small carnation without a cap or a needle and
hang a figure on it through any hole. Notice the shape
should swing freely on the stud.
Take a plumb line, consisting of a thin thread and weight, and throw it over
thread through the stud so that it indicates the vertical direction is not
suspended figure. Mark the vertical direction on the figure with a pencil
Remove the figure, hang it in any other hole and again with
Using a plumb line and a pencil, mark the vertical direction of the thread on it.
The intersection point of the vertical lines will indicate the position of the center of gravity
this figure.
Pass a thread through the center of gravity you found, at the end of which
a knot is made, and hang the figure on this thread. The figure must be kept
almost horizontal. The more accurately the experience is done, the more horizontal it will be.
keep figure.
Task 5.
Determine the center of gravity of the hoop.
Take a small hoop (for example, a hoop) or make a ring out of
flexible twig, from a narrow strip of plywood or hard cardboard. hang up
it on a stud and lower the plumb line from the hanging point. When the plumb line
calm down, mark on the hoop the points of her touch to the hoop and between
stretch and fasten a piece of thin wire or fishing line with these points
(you need to pull hard enough, but not so much that the hoop changes its
Hang the hoop on a stud at any other point and do the same
most. The intersection point of the wires or lines will be the center of gravity of the hoop.
Note: the center of gravity of the hoop lies outside the substance of the body.
Tie a thread to the intersection of wires or lines and hang it on
her hoop. The hoop will be in indifferent equilibrium, since the center
the gravity of the hoop and the point of its support (suspension) coincide.
Task 6.
You know that the stability of the body depends on the position of the center of gravity and
on the size of the support area: the lower the center of gravity and the larger the support area,
the more stable the body.
With this in mind, take a bar or an empty matchbox and, placing it
alternately on paper in a box to the widest, to the middle and to the most
smaller side, circle each time with a pencil to get three different
support area. Calculate the size of each area in square centimeters
and write them down on paper.
Measure and record the height of the center of gravity of the box for all
three cases (the center of gravity of a matchbox lies at the intersection
diagonals). Conclude at what position of the boxes is the most
sustainable.
Task 7.
Sit on a chair. Place your feet vertically without slipping them under
seat. Sit completely straight. Try to stand up without bending forward
without stretching your arms forward and without moving your legs under the seat. you have nothing
succeed - you won't be able to get up. Your center of gravity, which is located somewhere
in the middle of your body, will not let you get up.
What condition must be met in order to get up? Gotta lean forward
or tuck your feet under the seat. When we get up, we always do both.
In this case, the vertical line passing through your center of gravity should
be sure to go through at least one of the soles of your feet or between them.
Then the balance of your body will be stable enough, you can easily
you can get up.
Well, now try to stand up, picking up dumbbells or an iron. Pull out
hands forward. You may be able to stand up without bending over or bending your legs under
Inertia. Exercise 1.
Put a postcard on the glass, and put a coin on the postcard
or checker so that the coin is above the glass. Hit the postcard
click. The postcard should fly out, and the coin (checker) should fall into the glass.
Task 2.
Place a double sheet of notebook paper on the table. For one half
sheet, put a stack of books at least 25 cm high.
Slightly lifting the second half of the sheet above the level of the table with both
hands, quickly pull the sheet towards you. The sheet should come out from under
books, and the books should stay where they are.
Put the book back on the sheet and pull it now very slowly. Books
will move with the sheet.
Task 3.
Take a hammer, tie a thin thread to it, but so that it
withstood the weight of a hammer. If one thread fails, take two
threads. Slowly lift the hammer up by the thread. The hammer will hang on
thread. And if you want to raise it again, but not slowly, but quickly
jerk, the thread will break (ensure that the hammer, falling, does not break
nothing underneath). The inertia of the hammer is so great that the thread does not
survived. The hammer did not have time to quickly follow your hand, remained in place, and the thread broke.
Task 4.
Take a small ball made of wood, plastic or glass. Make out
thick paper groove, put a ball in it. Move quickly across the table
groove, and then suddenly stop it. The inertia ball will continue
movement and roll, jumping out of the groove.
Check where the ball will roll if:
a) pull the chute very quickly and stop it abruptly;
b) pull the chute slowly and stop abruptly.
Task 5.
Cut the apple in half, but not all the way through, and let it hang
Now hit with the blunt side of the knife with the apple hanging on top of it on
something hard, like a hammer. Apple, moving on
inertia, will be cut and split into two halves.
The same thing happens when wood is chopped: if it was not possible
split a block of wood, they usually turn it over and, with all their strength, hit it with a butt
an ax on a solid support. Churbak, continuing to move by inertia,
is planted deeper on the ax and splits in two.
Physics"
Atphysics teacher:
Gorsheneva Natalya Ivanovna
2011
G
The role of experiment in teaching physics.
Already in the definition of physics as a science, there is a combination of both theoretical and practical parts in it. It is very important that in the process of teaching physics the teacher be able to demonstrate to his students the interconnection of these parts as fully as possible. After all, when students feel this relationship, they will be able to give a correct theoretical explanation to many of the processes taking place around them in everyday life, in nature.
Without experiment there is not, and cannot be, a rational teaching of physics; mere verbal teaching of physics inevitably leads to formalism and rote memorization. The teacher's first thoughts should be directed to the fact that the student sees the experiment and does it himself, sees the device in the teacher's hands and holds it in his own hands.
A learning experiment is a means of learning in the form of experiments specially organized and conducted by a teacher and a student.
Objectives of the educational experiment:
Solving the main educational tasks;
Formation and development of cognitive and mental activity;
Polytechnic training;
Formation of students' worldview.
Cognitive (the basics of sciences are mastered in practice);
Educational (formation of a scientific worldview);
Developing (develops thinking and skills).
Types of physical experiments.
What forms of practical training can be offered in addition to the teacher's story? First of all, of course, this is the observation by students of the demonstration of experiments conducted by the teacher in the classroom when explaining new material or when repeating the past, it is also possible to offer experiments conducted by the students themselves in the classroom during lessons in the process of frontal laboratory work under the direct supervision of the teacher. You can also suggest: 1) experiments conducted by the students themselves in the classroom during a physical workshop; 2) experiments-demonstrations conducted by students when answering; 3) experiments conducted by students outside the school on the teacher's homework; 4) observations of short-term and long-term phenomena of nature, technology and everyday life, carried out by students at home on special assignments from the teacher.
What can be said about the above forms of education?
Demo Experiment is one of the components of the educational physical experiment and is a reproduction of physical phenomena by a teacher on a demonstration table using special devices. It belongs to the illustrative empirical methods learning. The role of a demonstration experiment in teaching is determined by the role that the experiment plays in physics and science as a source of knowledge and a criterion for its truth, and its possibilities for organizing the educational and cognitive activity of students.
The value of the demonstration physical experiment lies in the fact that:
Students get acquainted with the experimental method of cognition in physics, with the role of experiment in physical research (as a result, they form a scientific worldview);
Students develop some experimental skills: observe phenomena, put forward hypotheses, plan an experiment, analyze results, establish relationships between quantities, draw conclusions, etc.
The demonstration experiment, being a means of visualization, contributes to the organization of students' perception educational material, its understanding and memorization; allows for polytechnic education of students; promotes an increase in interest in the study of physics and the creation of motivation for learning. But when a teacher conducts a demonstration experiment, the main activity is performed by the teacher himself and, at best, by one or two students, the rest of the students only passively observe the experiment conducted by the teacher, while they themselves do nothing with their own hands. Therefore, it is necessary to have an independent experiment of students in physics.
Laboratory studies.
When teaching physics in secondary school, experimental skills are formed when they themselves assemble installations, measure physical quantities, and perform experiments. Laboratory studies arouse great interest among students, which is quite natural, since in this case the student learns about the world around him based on his own experience and his own feelings.
The significance of laboratory classes in physics lies in the fact that students form ideas about the role and place of the experiment in cognition. When performing experiments, students develop experimental skills, which include both intellectual and practical skills. The first group includes skills: to determine the purpose of the experiment, to put forward hypotheses, to select instruments, to plan an experiment, to calculate errors, to analyze results, to draw up a report on the work done. The second group includes skills: to assemble an experimental setup, to observe, measure, experiment.
In addition, the significance of a laboratory experiment lies in the fact that when it is performed, students develop such important personal qualities as accuracy in working with instruments; observance of cleanliness and order in the workplace, in the records that are made during the experiment, organization, perseverance in obtaining results. They form a certain culture of mental and physical labor.
In the practice of teaching physics at school, three types of laboratory classes have developed:
Frontal laboratory work in physics;
Physical workshop;
Home experimental work in physics.
Performing independent laboratory work.
Frontal laboratory work - this is a type of practical work when all students in the class simultaneously perform the same type of experiment using the same equipment. Frontal laboratory work is most often performed by a group of students consisting of two people, sometimes it is possible to organize individual work. Here a difficulty arises: not always in the school physics office there is a sufficient number of sets of instruments and equipment for such work. Old equipment falls into disrepair, and, unfortunately, not all schools can afford to purchase new ones. Yes, and there is no escape from the time limit. And if one of the teams does not succeed, some device does not work, or something is missing, then they start asking the teacher for help, distracting others from doing laboratory work.
In grades 9-11, a physical workshop is held.
Physical workshop is carried out with the aim of repeating, deepening, expanding and generalizing the knowledge gained from various topics of the physics course; development and improvement of students' experimental skills through the use of more sophisticated equipment, more complex experiments; the formation of their independence in solving problems related to the experiment. A physical practice is held, usually at the end school year, sometimes - at the end of the first and second half of the year and includes a series of experiments on a particular topic. Students perform the work of a physical workshop in a group of 2-4 people using various equipment; in the following classes there is a change of work, which is done according to a specially drawn up schedule. When scheduling, take into account the number of students in the class, the number of workshops, the availability of equipment. Two academic hours are assigned to each work of the physical workshop, which requires the introduction of double lessons in physics into the schedule. This presents difficulties. For this reason, and due to the lack of necessary equipment, one-hour work of a physical workshop is practiced. It should be noted that two-hour work is preferable, since the work of the workshop is more difficult than frontal laboratory work, they are performed on more sophisticated equipment, and the proportion of students' independent participation is much larger than in the case of frontal laboratory work.
For each work, the teacher must draw up an instruction that should contain: name, purpose, list of instruments and equipment, a brief theory, a description of instruments unknown to students, a work plan. After completing the work, students must submit a report that should contain: the name of the work, the purpose of the work, a list of instruments, a diagram or drawing of the installation, a work execution plan, a table of results, formulas by which the values \u200b\u200bof were calculated, calculation of measurement errors, conclusions. When evaluating the work of students in the workshop, one should take into account their preparation for work, a report on the work, the level of formation of skills, understanding theoretical material used methods of experimental research.
And what will happen if the teacher invites the students to perform an experiment or conduct an observation outside the school, that is, at home or on the street? home experiments should not require the use of any instruments and significant material costs. These should be experiments with water, air, with objects that are in every home. Someone may doubt the scientific value of such experiments, of course, it is minimal there. But is it really bad if the child himself can check the law or phenomenon discovered many years before him? There is no benefit for humanity, but what is it for a child! Experience is a creative task, doing something on their own, the student, whether he wants it or not, but he will think: how easier it is to conduct an experiment, where he met with a similar phenomenon in practice, where else it can be useful this phenomenon. Here it should be noted that children learn to distinguish physical experiments from all sorts of tricks, not to confuse one with the other.
Home experimental work. Home laboratory work is the simplest independent experiment that is performed by students at home, outside of school, without direct control from the teacher over the progress of work.
The main tasks of this type of experimental work are:
Formation of the ability to observe physical phenomena in nature and in everyday life;
Formation of the ability to perform measurements with the help of measuring instruments used in everyday life;
Formation of interest in experiment and in the study of physics;
Formation of independence and activity.
Home laboratory work can be classified depending on the equipment used in their performance:
Works that use household items and improvised materials (measuring cup, tape measure, household scales, etc.);
Works in which home-made devices are used (lever scales, electroscope, etc.);
What does a child need to conduct an experiment at home? First of all, that's probably enough. detailed description experience, indicating the necessary items, where in a form accessible to the child it is said what needs to be done, what to pay attention to. In addition, the teacher is obliged to conduct a detailed briefing.
Requirements for home experiments. First of all, it is, of course, safety. Since the experiment is carried out by the student at home on his own, without the direct supervision of the teacher, the experiment should not contain any chemicals and objects that pose a threat to the health of the child and his home environment. The experiment should not require any significant material costs from the student; during the experiment, objects and substances that are in almost every home should be used: dishes, jars, bottles, water, salt, and so on. An experiment performed by schoolchildren at home should be simple in execution and equipment, but, at the same time, be valuable in the study and understanding of physics in childhood, be interesting in content. Since the teacher does not have the opportunity to directly control the experiment performed by students at home, the results of the experiment should be appropriately formalized (approximately as is done when performing frontal laboratory work). The results of the experiment conducted by the students at home should be discussed and analyzed in the lesson. Students' work should not be a blind imitation of established patterns; they should contain the broadest manifestation of their own initiative, creativity, and search for something new. Based on the foregoing, we briefly formulate the requirements for home experimental tasks requirements:
Safety during the conduct;
Minimum material costs;
Ease of implementation;
Ease of subsequent control by the teacher;
The presence of creative coloring.
A home experiment can be given after the topic has been covered in class. Then the students will see with my own eyes and be convinced of the validity of the theoretically studied law or phenomenon. At the same time, the knowledge obtained theoretically and tested in practice will be firmly deposited in their minds.
And vice versa, you can set a homework task, and after completing it, carry out an explanation of the phenomenon. Thus, students can create problem situation and move on to problem-based learning, which involuntarily gives rise to students' cognitive interest in the material being studied, ensures the cognitive activity of students in the course of learning, leads to the development creative thinking students. In this case, even if schoolchildren cannot explain the phenomenon they saw at home, they will listen with interest to the teacher's story.
Stages of the experiment:
Justification of the experiment.
Planning and conducting an experiment.
Evaluation of the result.
Formulation and justification of the hypothesis that can be used as the basis for the experiment.
Determining the purpose of the experiment.
Finding out the conditions necessary to achieve the goal of the experiment.
Planning an experiment, including answering the questions:
what observations to make
what quantities to measure
instruments and materials necessary for the experiments
the course of experiments and the sequence of their implementation
choice of the form for recording the results of the experiment
Selection of necessary instruments and materials
Installation collection.
Conducting an experiment, accompanied by observations, measurements and recording their results
Mathematical processing of measurement results
Analysis of the results of the experiment, formulation of conclusions
When conducting any experiment, it is necessary to remember the requirements for the experiment.
Experiment Requirements:
visibility;
short duration;
Persuasiveness, accessibility, reliability;
Security.
In addition to the above types of experiments, there are mental, virtual experiments (see Appendix), which are carried out in virtual laboratories and have great importance in the absence of equipment.
Psychologists note that complex visual material is remembered better than its description. Therefore, the demonstration of experiments is captured better than the teacher's story about physical experience.
The school is the most amazing laboratory, because the future is created in it! And what it will be depends on us teachers!
I believe that if a teacher in teaching physics uses an experimental method, in which students are systematically involved in the search for ways to solve issues and problems, then we can expect that the result of training will be the development of versatile, original, not constrained by narrow limits of thinking. A is the way to the development of high intellectual activity of students.
Appendix.
Classification of types of experiments.
Field
(excursions)
Home
School
Mental
Real
Virtual
Depending on quantity and size
laboratory
Practice
demonstration
By venue
By way of doing
Depending on the subject
Experiment
)physics teacher
State Autonomous Educational Institution Vocational School No. 3, Buzuluk
Pedsovet.su - thousands of materials for the daily work of a teacher
Experimental work on the development of students' skills vocational schools solve problems in physics.
Problem solving is one of the main ways to develop students' thinking, as well as to consolidate their knowledge. Therefore, after analyzing the current situation, when some students could not solve even an elementary problem, not only because of problems with physics, but also with mathematics. My task consisted of the mathematical side and the physical side.
In my work on overcoming the mathematical difficulties of students, I used the experience of teachers N.I. Odintsova (Moscow, Moscow State Pedagogical University) and E.E. Yakovets (Moscow, secondary school No. 873) with correction cards. The cards are modeled after cards used in a math course, but are focused on a physics course. The cards were made on all issues of the mathematics course that cause difficulties for students in physics lessons (“Conversion of units of measurement”, “Using the properties of a degree with an integer indicator”, “Expressing a quantity from a formula”, etc.)
Correction cards have similar structures:
rule → pattern → task
definition, action → pattern → task
actions → sample → task
Correction cards are used in the following cases:
For preparation for tests and as material for self-study.
Students in class or extra lesson in physics before the test, knowing their gaps in mathematics, they can get a specific card on a poorly mastered mathematical question, work out and eliminate the gap.
To work on the mathematical mistakes made in the control.
After checking the control work, the teacher analyzes the mathematical difficulties of the students and draws their attention to the mistakes made, which they eliminate in the lesson or in the additional lesson.
To work with students in preparation for the exam and various olympiads.
When studying the next physical law, and at the end of studying a small chapter or section, I suggest that students for the first time jointly, and then independently (homework) fill out table No. 2. At the same time, I give an explanation that such tables will help us in solving problems.
Table number 2
Name
physical quantity
To this end, in the first lesson on solving problems, I show students how to use this table using a specific example. And I propose an algorithm for solving elementary physical problems.
Determine which quantity is unknown in the problem.
Using table No. 1, find out the designation, units of measurement of the quantity, as well as the mathematical law connecting the unknown quantity and the quantities specified in the problem.
Check the completeness of the data required to solve the problem. If there are not enough, use the appropriate values from the lookup table.
Checkout short note, analytical solution and numerical answer of the problem in conventional notation.
I draw the attention of students that the algorithm is quite simple and universal. It can be applied to the solution of an elementary problem from almost any section of school physics. Later, elementary tasks will be included as auxiliary tasks in higher-level tasks.
There are a lot of such algorithms for solving problems on specific topics, but it is almost impossible to remember them all, therefore it is more expedient to teach students not the methods of solving individual problems, but the method of finding their solution.
The process of solving a problem consists in the gradual correlation of the condition of the problem with its requirement. When starting to study physics, students do not have the experience of solving physical problems, but some elements of the process of solving problems in mathematics can be transferred to solving problems in physics. The process of teaching students the ability to solve physical problems is based on the conscious formation of their knowledge about the means of solving.
To this end, in the first lesson on problem solving, students should be introduced to a physical problem: to present the condition of the problem to them as a specific plot situation in which some physical phenomenon occurs.
Of course, the process of developing the ability of students to independently solve problems begins with the development of their ability to perform simple operations. First of all, students should be taught to correctly and completely write down a short record (“Given”). To do this, they are invited to single out the structural elements of the phenomenon from the text of several tasks: the material object, its initial and final states, the influencing object and the conditions for their interaction. According to this scheme, first the teacher, and then each of the students independently analyze the conditions of the received tasks.
Let us illustrate what has been said with examples of the analysis of the conditions of the following physical problems (Table No. 3):
An ebony ball, negatively charged, is suspended from a silk thread. Will the force of its tension change if the second identical but positively charged ball is placed at the point of suspension?
If a charged conductor is covered with dust, then it quickly loses its charge. Why?
Between two plates placed horizontally in vacuum at a distance of 4.8 mm from each other, a negatively charged oil droplet weighing 10 ng is in equilibrium. How many "excess" electrons does a drop have if a voltage of 1 kV is applied to the plates?
Table No. 3
Structural elements of the phenomenon
Unmistakable Finding structural elements phenomena in the text of the task by all students (after analyzing 5-6 tasks) allows you to move on to the next part of the lesson, which aims to assimilate the sequence of operations for students. Thus, in total, students analyze about 14 tasks (without completing the solution), which turns out to be sufficient for learning to perform the action “highlighting the structural elements of a phenomenon”.
Table No. 4
Card - prescription
Task: express the structural elements of the phenomenon in
physical concepts and values
indicative signs
- Replace the material object specified in the problem with the corresponding idealized object Express the characteristics of the initial object using physical quantities. Replace the influencing object specified in the task with the corresponding idealized object. Express the characteristics of the influencing object using physical quantities. Express the characteristics of the interaction conditions using physical quantities. Express the characteristics of the final state of a material object using physical quantities.
Next, students learn to express the structural elements of the phenomenon under consideration and their characteristics in the language physical science, which is extremely important, since all physical laws are formulated for certain models, and for a real phenomenon described in the problem, an appropriate model must be built. For example: "a small charged ball" - a point charge; "thin thread" - the mass of the thread is negligible; "silk thread" - no charge leakage, etc.
The process of forming this action is similar to the previous one: first, the teacher, in a conversation with students, shows with 2-3 examples how to perform it, then the students perform the operations on their own.
The action "drawing up a plan for solving the problem" is formed by students immediately, since the components of the operation are already known to students and mastered by them. After showing a sample of performing an action, each student is given a card for independent work - the instruction “Drawing up a plan for solving the problem”. The formation of this action is carried out until it is performed unmistakably by all students.
Table number 5
Card - prescription
"Drawing up a plan for solving the problem"
Operations in progress
- Determine what characteristics of the material object have changed as a result of the interaction. Find out the reason for this change in the state of the object. Write down the cause-and-effect relationship between the impact under given conditions and the change in the state of the object in the form of an equation. Express each term of the equation in terms of physical quantities characterizing the state of the object and the conditions of interaction. Select the desired physical quantity.
Express the required physical quantity in terms of other known ones.
The fourth and fifth stages of problem solving are carried out traditionally. After mastering all the actions that make up the content of the method for finding a solution to a physical problem, a complete list of them is written out on a card, which serves as a guide for students in independently solving problems for several lessons.
For me, this method is valuable in that, assimilated by students when studying one of the sections of physics (when it becomes a style of thinking), it is successfully applied in solving problems of any section.
During the experiment, it became necessary to print algorithms for solving problems on separate sheets for students to work not only in the lesson and after the lesson, but also at home. As a result of the work on the development of subject competence in solving problems, a folder of didactic material was compiled for solving problems that any student could use. Then, together with the students, several copies of such folders were made for each table.
The use of an individual approach helped to form in students the most important components learning activities- self-esteem and self-control. The correctness of the solution of the problem was checked by the teacher and students - consultants, and then more and more students began to help each other more often, involuntarily drawn into the process of solving problems.
Vibrations and waves.
Optics.
Tasks for independent work.
Task 1. Hydrostatic weighing.
Equipment: wooden ruler length 40 cm, plasticine, a piece of chalk, a measuring cup with water, threads, a razor blade, a tripod with a holder.
The task.
Measure
- plasticine density;
- chalk density;
- mass of wooden ruler.
Notes:
- It is advisable not to wet a piece of chalk - it can fall apart.
- The density of water is considered equal to 1000 kg / m 3
Problem 2. Specific heat of dissolution of hyposulfite.
When dissolving hyposulfite in water, the temperature of the solution decreases greatly.
Measure the specific heat of solution of the given substance.
The specific heat of dissolution is understood as the amount of heat required to dissolve a unit mass of a substance.
The specific heat capacity of water is 4200 J/(kg × K), the density of water is 1000 kg/m 3 .
Equipment: calorimeter; beaker or measuring cup; scales with weights; thermometer; crystalline hyposulfite; warm water.
Problem 3. Mathematical pendulum and free fall acceleration.
Equipment: a tripod with a foot, a stopwatch, a piece of plasticine, a ruler, a thread.
The task: Measure free fall acceleration with a mathematical pendulum.
Problem 4. The refractive index of the lens material.
The task: Measure the refractive index of the glass that the lens is made from.
Equipment: a biconvex lens on a stand, a light source (a light bulb on a stand with a current source and connecting wires), a screen on a stand, a caliper, a ruler.
Problem 5. "Vibrations of the rod"
Equipment: a tripod with a foot, a stopwatch, a knitting needle, an eraser, a needle, a ruler, a plastic cork from plastic bottle.
- Explore the dependence of the oscillation period of the resulting physical pendulum on the length of the upper part of the spoke. Plot the resulting dependency. Check the feasibility of formula (1) in your case.
- Determine with the maximum possible accuracy the minimum period of oscillation of the resulting pendulum.
- Determine the value of the free fall acceleration.
Task 6. Determine with the greatest possible accuracy the resistance of the resistor.
Equipment: a current source, a resistor with a known resistance, a resistor with an unknown resistance, a cup (glass, 100 ml), a thermometer, a watch (you can use your wrist), graph paper, a piece of foam.
Task 7. Determine the friction coefficient of the bar on the table.
Equipment: bar, ruler, tripod, threads, weight of known mass.
Task 8. Determine the weight of a flat figure.
Equipment: flat figure, ruler, weight.
Problem 9. Investigate the dependence of the speed of the jet flowing out of the vessel on the height of the water level in this vessel .
Equipment: tripod with clutch and foot, glass burette with scale and rubber tube; spring clip; screw clamp; stopwatch; funnel; cuvette; a glass of water; sheet of graph paper.
Task 10. Determine the temperature of water at which its density is maximum.
Equipment: a glass of water, at a temperature t = 0 °C; metal stand; thermometer; spoon; clock; small glass.
Task 11. Determine the strength of the gap T threads, mg< T
.
Equipment: bar whose length 50 cm; thread or thin wire; ruler; cargo of a known mass; tripod.
Task 12. Determine the coefficient of friction of a metal cylinder, the mass of which is known, on the surface of the table.
Equipment: two metal cylinders of approximately the same mass (the mass of one of them is known ( m = 0.4 - 0.6 kg)); length ruler 40 - 50 cm; Bakushinsky dynamometer.
Problem 13. Explore the contents of the mechanical "black box". Determine the characteristics of the rigid body enclosed in the "box".
Equipment: dynamometer, ruler, graph paper, "black box" - a closed jar, partially filled with water, in which there is a solid body with a rigid wire attached to it. The wire exits the can through a small hole in the lid.
Problem 14. Determine the density and specific heat of an unknown metal.
Equipment: a calorimeter, a plastic cup, a bath for developing photographs, a measuring cylinder (beaker), a thermometer, threads, 2 cylinders of an unknown metal, a vessel with hot ( t g \u003d 60 ° -70 °) and cold ( t x \u003d 10 ° - 15 °) with water. Specific heat capacity of water c in \u003d 4200 J / (kg × K).
Problem 15. Determine Young's modulus of steel wire.
Equipment: tripod with two legs for attaching equipment; two steel rods; steel wire (diameter 0.26 mm); ruler; dynamometer; plasticine; pin.
Note. The stiffness coefficient of the wire depends on the Young's modulus and the geometric dimensions of the wire as follows k = ES/l, where l is the length of the wire, a S is the area of its cross section.
Task 16. Determine the concentration table salt in the aqueous solution given to you.
Equipment: glass jar volume 0.5 l; vessel with aqueous solution table salt of unknown concentration; alternating current source with adjustable voltage; ammeter; voltmeter; two electrodes; connecting wires; key; set of 8
weights of table salt; graph paper; fresh water container.
Task 17. Determine the resistance of a millivoltmeter and a milliammeter for two measurement ranges.
Equipment: millivoltmeter ( 50/250 mV), milliammeter ( 5/50 mA), two connecting wires, copper and zinc plates, pickles.
Problem 18. Determine the density of the body.
Equipment: body irregular shape, metal rod, ruler, tripod, vessel with water, thread.
Task 19. Determine the resistances of the resistors R 1, ..., R 7, ammeter and voltmeter.
Equipment: battery, voltmeter, ammeter, connecting wires, switch, resistors: R 1 - R 7.
Problem 20. Determine the coefficient of spring stiffness.
Equipment: spring, ruler, sheet of graph paper, bar, weight 100 g.
Attention! Do not hang a load on the spring, as this will exceed the elastic limit of the spring.
Task 21. Determine the sliding friction coefficient of the match head on the rough surface of the matchbox.
Equipment: box of matches, dynamometer, weight, sheet of paper, ruler, thread.
Problem 22. The part of the fiber optic connector is a glass cylinder (refractive index n= 1.51), which has two round cylindrical channels. The ends of the part are sealed. Determine channel spacing.
Equipment: connector detail, graph paper, magnifier.
Problem 23. "Black vessel". A body is lowered into a "black vessel" with water on a thread. Find the density of the body ρ m , its height l the water level in the vessel with the submerged body ( h) and when the body is outside the liquid ( h o).
Equipment. "Black Vessel", dynamometer, graph paper, ruler.
Density of water 1000 kg/m3. Vessel depth H = 32 cm.
Problem 24. Friction. Determine the sliding friction coefficients of wooden and plastic rulers on the surface of the table.
Equipment. Tripod with foot, plumb line, wooden ruler, plastic ruler, table.
Problem 25. Clockwork toy. Determine the energy stored by the spring of a clockwork toy (car) with a fixed “winding” (number of turns of the key).
Equipment: a clockwork toy of known mass, a ruler, a tripod with a foot and a clutch, an inclined plane.
Note. Wind up the toy so that its run does not exceed the length of the table.
Problem 26. Determining the density of bodies. Determine the density of the load (rubber bung) and the lever (wooden lath) using the proposed equipment.
Equipment: cargo of known mass (marked cork); lever (wooden rail); cylindrical glass ( 200 - 250 ml); a thread ( 1m); wooden ruler, a vessel with water.
Problem 27. We study the movement of the ball.
Raise the ball to a certain height above the table surface. Let's release it and observe its movement. If the collisions were absolutely elastic (sometimes they say elastic), then the ball would always jump to the same height. In reality, the height of the jumps is constantly decreasing. The time interval between successive jumps also decreases, which is clearly noticeable by ear. After some time, the jumps stop and the ball remains on the table.
1 task - theoretical.
1.1. Determine the proportion of lost (energy loss factor) energy after the first, second, third bounce.
1.2. Get the dependence of time on the number of bounces.
2 task - experimental.
2.1. Direct method, using a ruler, determine the coefficient of energy loss after the first, second, third impact.
It is possible to determine the energy loss coefficient using a method based on measuring the total time of the ball's movement from the moment it is thrown from a height H to the moment the bounces stop. To do this, you have to establish the relationship between the total travel time and the energy loss coefficient.
2.2. Determine the energy loss factor using a method based on measuring the total time of the ball's movement.
3. Errors.
3.1. Compare the measurement errors of the energy loss factor in paragraphs 2.1 and 2.2.
Problem 28.
- Find the mass of the test tube given to you and its outer and inner diameters.
- Calculate theoretically at what is the smallest height h min and the largest height h max of water poured into a test tube, it will float steadily in vertical position, and find numerical values using the results of the first paragraph.
- Determine h min and h max experimentally and compare with the results of point 2.
Equipment. A test tube of unknown mass with a glued scale, a vessel with water, a glass, a sheet of graph paper, a thread.
Note. It is forbidden to peel off the scale from the test tube!
Problem 29. Angle between mirrors. Determine the dihedral angle between mirrors with the greatest accuracy.
Equipment. Two mirror system, measuring tape, 3 pins, cardboard sheet.
Problem 30. Spherical segment.
A spherical segment is a body bounded by a spherical surface and a plane. Using this equipment, build a graph of the dependence of the volume V spherical segment of unit radius r = 1 from his height h.
Note. The formula for the volume of a spherical segment is not supposed to be known. Take the density of water equal to 1.0 g/cm 3 .
Equipment. A glass of water, a tennis ball of known mass m with a puncture, a syringe with a needle, a sheet of graph paper, adhesive tape, scissors.
Problem 31. Snow with water.
Determine the mass fraction of snow in the mixture of snow and water at the time of issue.
Equipment. A mixture of snow and ice, a thermometer, a watch.
Note. The specific heat capacity of water c = 4200 J/(kg × °C), the specific heat of ice melting λ = 335 kJ/kg.
Problem 32. Adjustable "black box".
In the "black box", which has 3 outputs, an electrical circuit is assembled, consisting of several resistors with a constant resistance and one variable resistor. The resistance of the variable resistor can be changed from zero to a certain maximum value R o using the adjusting knob brought out.
Using an ohmmeter, examine the circuit of the "black box" and, assuming that the number of resistors in it is minimal,
- draw a diagram of an electrical circuit enclosed in a "black box";
- calculate the resistance of fixed resistors and the value of R o ;
- evaluate the accuracy of the resistance values you calculated.
Problem 33. Measurement of electrical resistances.
Determine the resistance of the voltmeter, battery and resistor. It is known that a real battery can be represented as an ideal one, connected in series with some resistor, and a real voltmeter - as an ideal one, in parallel with which a resistor is connected.
Equipment. Battery, voltmeter, resistor with unknown resistance, resistor with known resistance.
Problem 34. Weighing ultra-light loads.
Using the proposed equipment, determine the mass m of a piece of foil.
Equipment. A jar of water, a piece of Styrofoam, a set of nails, wooden toothpicks, a ruler with millimeter divisions or graph paper, a sharpened pencil, foil, napkins.
Problem 35.
Determine the current-voltage characteristic (CVC) of the "black box" ( CJ). Describe the method of taking the CVC and build its graph. Estimate the errors.
Equipment. CJ, limiting resistor with a known resistance R, multimeter in voltmeter mode, adjustable current source, connecting wires, graph paper.
Attention. connect CJ to the current source bypassing the limiting resistor is strictly prohibited.
Problem 36. Soft spring.
- Experimentally investigate the dependence of the elongation of a soft spring under the action of its own weight on the number of coils of the spring. Give a theoretical explanation of the relationship found.
- Determine the coefficient of elasticity and the mass of the spring.
- Investigate the dependence of the period of oscillation of the spring on its number of turns.
Equipment: a soft spring, a tripod with a foot, a tape measure, a clock with a second hand, a ball of plasticine with a mass m = 10 g, graph paper.
Problem 37. Wire Density.
Determine the density of the wire. Breaking the wire is not allowed.
Equipment: piece of wire, graph paper, thread, water, vessel.
Note. Density of water 1000 kg/m3.
Problem 38. Friction coefficient.
Determine the sliding friction coefficient of the bobbin material on wood. The axis of the bobbin must be horizontal.
Equipment: bobbin, thread length 0.5 m, wooden ruler fixed at an angle in a tripod, graph paper.
Note. During the work it is forbidden to change the position of the ruler.
Problem 39. Share of mechanical energy.
Determine the fraction of mechanical energy lost by the ball when falling without initial velocity from a height 1m.
Equipment: tennis ball, ruler length 1.5 m, sheet of white paper format A4, sheet of carbon paper, glass plate, ruler; brick.
Note: for small deformations of the ball, one can (but not necessarily) consider Hooke's law to be valid.
Problem 40. A vessel with water "black box".
The "black box" is a vessel with water, into which a thread is lowered, on which two weights are fixed at some distance from each other. Find the masses of the loads and their densities. Estimate the size of the loads, the distance between them and the level of water in the vessel.
Equipment: "black box", dynamometer, graph paper.
Problem 41. Optical "black box".
The optical "black box" consists of two lenses, one of which is converging and the other diverging. Determine their focal lengths.
Equipment: a tube with two lenses (optical "black" box), a light bulb, a current source, a ruler, a screen with a sheet of graph paper, a sheet of graph paper.
Note. The use of light from a distant source is allowed. It is not allowed to bring the light bulb close to the lenses (that is, closer than the racks allow).
The effectiveness of using experimental tasks in the classroom is largely determined by their manufacturability, unpretentiousness in equipment, and the breadth of the phenomena under consideration. Based on the simplest equipment and even on household items, the experimental task brings physics closer to us, turning it in the minds of students from an abstract system of knowledge into science, studying the “world around us”.
Mechanics
Task 1. Friction coefficient
The task. Measure the coefficient of sliding friction of a wooden block on the surface of the board (ruler).
Equipment: bar, board, tripod with foot, ruler 30 (40) long cm.
Possible way solutions. We put the bar on the plank, in accordance with Figure 4. Gradually raising one end of the board, we get an inclined plane and achieve uniform sliding of the bar. Since the static friction force is much more power sliding friction, it is necessary to push the bead a little at the beginning of the slide. Use a tripod to fix the desired tilt. We measure the height but and the length of the base of the inclined plane b.
Measurements and error analysis:
The experiment is repeated several times. In this case, this must be done mainly because it is difficult to achieve precisely uniform sliding of the bar along the plane. The results are entered in table 2.
table 2
Measurement errors
|
a, see |
Yes, see |
(Yes) 2 ,cm 2 |
in, cm |
Db, cm |
(Db) 2 ,cm 2 |
|
<a>=12,2 |
Y( a) 2 = 1,81 |
Y( b) 2 = 0,32 |
In addition to random errors, the general error, of course, also includes the usual errors of departure: Yes = Db = 0.5 cm.This amounts to:
Thus, we get:
a = 12.2 ± 1.1 cm, d = 8.6%
b = 27.4 ± 0.7 cm, d = 2.6%
According to the results of the first experience:
The final result of the measurement of the coefficient of friction:
m = 0.46 ± 0.05 d = 10.9%
Task 2. Measuring the height of a house
The task. Imagine that you were asked to use an empty tin can and a stopwatch to measure the height of a house. Would you be able to complete the task? Tell us how to proceed.
Prompt. If the jar is thrown from the roof of the house, then the sound of the jar hitting the earth's surface will be clearly audible.
Solution. Standing on the roof of the house, you need to release the jar from your hands, while simultaneously pressing the start button of the stopwatch. When you hear the sound of the jar hitting the ground, you should stop the stopwatch. Stopwatch indications t are made up of the time of the fall of the bank t 1 and time t 2 , during which the sound of its impact on the earth's surface will reach the observer.
The first time is related to the height of the house h in the following way:
while the relationship between h and t 2 looks like
where from- the speed of sound, which in the calculations we set equal to 340 m/s.
Defining t 1 and t 2 of these expressions and substituting their values into a formula relating t 1 , t 2 and t, we get the irrational equation
From which you can find the height of the house.
In an approximate calculation (especially if the house is not high), the second term on the left can be considered small and discarded. Then
Molecular physics
Task 3. Pencil
The task. Estimate the mechanical work that must be done in order to evenly raise the pencil floating in the vessel to the level of its lower end touching the surface of the water. Consider the position of the pencil vertical. Density of water from 0 = 1000 kg/m 3 .
Equipment: round pencil, almost full water bottle, ruler.
Possible solution. We lower the pencil into the bottle - it will float like a float, in accordance with Figure 5. Let L- the length of the entire pencil, V- its volume, h is the length of the submerged part of the pencil, V 1 - its volume, S- sectional area and d is the diameter of the pencil. Find the average density of a pencil from from the condition of body swimming:
from 0 gSh= cgSL, where from= from 0 hL.
Suppose we are pulling a pencil out of the water at a constant speed using a dynamometer. When the pencil floats freely, the dynamometer reads zero. If the pencil is completely pulled out of the water, then the dynamometer will show a force equal to the weight R pencil:
F = P = mg = cgV = c0hLgSL = c0hgрd24
It turns out that the readings of the dynamometer when pulling a pencil out of the water change from 0 to P according to a linear law, in accordance with Figure 6. At the same time mechanical work BUT will be equal to the area of the selected triangle:
A= 12Ph= from 0 h 2grd 2 8.
For example, when h= 13,4 cm And d = 7,5 mm work is about 0.004 J.
Problem 4. Alloy
The task. Determine the percentage (by weight) of tin in the tin-lead solder. Assume that the volumes of lead and tin in the alloy are conserved. Lead Density from c = 11350 kg/m 3 , tin from 0 = 7300 kg/m 3 .
Equipment: ruler, weight (nut), cylindrical piece of solder, caliper or micrometer. Possible solution. This task is similar to the task of Archimedes to determine the proportion of gold in the royal crown. However, for experiments, tin-lead solder is easier to get than a crown.
By measuring the diameter of a piece of solder D and its length L, find the volume of a cylindrical piece of solder:
V =pD 2 L 4
We determine the mass of solder by making a balance scale. To do this, balance the ruler on the edge of the table (on a pencil, on a ballpoint pen, etc.). Then, using a nut of known mass, we balance a piece of solder on the ruler and, using the equality of the moments of forces, we find the mass of the solder m. Let's write the obvious equalities for the masses, volumes and densities of lead and tin:
m = m c +m o = ccV c +c o V o , V = V c +V o .
Solving these equations together, we find the volume of tin, its mass and its share in the total mass:
V o = rh o cV?mrh o c?rh oo , mo = with o V o , m o m = rh oo V o m
Problem 5. Surface tension
The task. Determine the coefficient of surface tension of water.
Equipment: a plate, water, a spoon, a ruler, a piece of even aluminum wire 15-20 long cm and density 2700 kg/m 3 , micrometer, alcohol, cotton wool.
Possible solution. Pour an almost full plate of water. We put a wire on the edge of the plate so that one end of it touches the water, and the other is outside the plate. The wire performs two functions: it is a balance and is analogous to a wire frame, which is usually pulled out of the water to measure surface tension. Depending on the water level, different positions of the wire can be observed. The most convenient for calculations and measurements is the horizontal arrangement of the wire at a water level of 1-1.5 mm below the rim of the plate, as shown in Figure 7. With a spoon, you can adjust the level by adding or draining water. The wire should be pulled out of the plate until the film of water under the wire begins to break. In this extreme position, the film has a height of 1.5-2 mm, and we can say that the surface tension forces applied to the wire are directed almost vertically downwards.
Let be m- mass of wire, L=L 1 + L 2 - wire length, m/L- mass per unit length of the wire. Let us write down the equilibrium condition for the wire relative to the edge of the plate, i.e. equality of the moments of forces:
F p (L 1 ?x 2)+m 1 gL 12 = m 2 gL 22 .
Substitute here the surface tension force F p =2x at, mass
m 1 =L 1 ml, m 2 = L 2 ml, m= cV= cd 2 L 4
and express the coefficient of surface tension at. Measurements and calculations will be simplified if the water wets the entire length L 1 . Finally we get
at= cd 2 g 8((LL 1 ?1) 2 ?1).
Quantities L And L 1 are measured with a ruler, and the wire diameter d- micrometer.
For example, when L = 15 cm, L 1 = 5,4 cm, d = 1,77 mm we get O = 0,0703 N/m, which is close to the tabular value of 0.0728 N/m.
Task 6. Humidity
The task. Determine the relative humidity in the room.
Equipment: a glass room thermometer, a household refrigerator, a table of pressures of saturated water vapor at various temperatures.
Possible solution. In the conventional method of measuring humidity, the object is cooled below the dew point and it "fogs". Let's do the opposite. The temperature in the refrigerator (about +5 ° C) is well below the room air dew point. Therefore, if you take a chilled glass thermometer out of the refrigerator, it will immediately "fog" - the glass case will become opaque from moisture. Then the thermometer will begin to heat up, and at some point the condensed moisture on it will evaporate - the glass will become transparent. This is the dew point temperature, from which, using the table, you can calculate the relative humidity.
Task 7. Evaporation
The task. Pour an almost full glass of water and put it in a room in a warm place - so that the water evaporates faster. Measure the initial water level with a ruler and record the start time of the experiment. After a few days, the water level will drop due to evaporation. Measure the new water level and record the end time of the experiment. Determine the mass of evaporated water. On average, how many molecules are ejected from the surface of the water in 1 second? Approximately how many molecules are on the surface of water in a glass? Compare these two numbers. Take the diameter of a water molecule equal to d 0 = 0,3 nm. Knowing the specific heat of vaporization, determine the rate of heat transfer ( j/s) water from environment.
Possible solution. Let be d- internal diameter of the glass, from- density of water, M is the molar mass of water, r- specific heat of vaporization, D h- decrease in water level over time t. Then the mass of evaporated water is
m= cv= from D hS= from D hrd 2 4.
This mass contains N=mN A /M molecules, where N A is the Avogadro constant. The number of molecules evaporated in 1 second is
N 1 = Nt= mN A Mt.
If S= pd 2/4 is the surface area of water in a glass, and S 0 = pd 2 0 /4 - the cross-sectional area of one molecule, then on the surface of the water in the glass is approximately
N 2 = SS 0 = (dd 0) 2 .
Water for evaporation receives the amount of heat per unit time
Qt= rmt.
If you make any calculations related to molecules, you always get interesting results. For example, let the time t= 5 days in a glass with a diameter d = 65 mm The water level has dropped by h = 1 cm. Then we get that 33 turned into steam G water, for 1 from evaporated N 1 \u003d 2.56 × 10 18 molecules, there were N 2 \u003d 4.69 × 1016 molecules, and 0.19 came from the environment Tue heat. The relationship is interesting N 1 /N 2? 54, from which it can be seen that for 1 from as many molecules evaporated as could be placed in a glass in 54 layers of water.
Task 8. Dissolution
The task. By pouring salt or sugar into boiling water, you will notice that the boiling stops for a short time due to a decrease in the temperature of the water. Determine the amount of heat required to dissolve 1 kg baking soda in room temperature water.
Equipment: home-made calorimeter, thermometer, water, soda, measuring cylinder (glass), known mass weight (nut mass 10 G), a plastic spoon.
Possible solution. The task includes an additional design task for the manufacture of a simple home-made calorimeter. For the inner vessel of the calorimeter, you should take an ordinary aluminum can with a volume of 0.33 liters. The top lid is removed from the can so that an aluminum glass is obtained (weighing only 12 G) with a rigid upper rim. A slot is made inside the upper rim so that the water completely pours out of the jar. The outer plastic shell is made on the basis of a plastic bottle with a volume of 1.5 l. The bottle is cut into three parts, the upper part is removed, and the middle and lower parts are inserted into each other with some force and tightly fix the inner aluminum can in a vertical position. (If there is no calorimeter, then experiments can be carried out in a disposable plastic cup, the mass and heat transfer of which can be neglected).
Beforehand, two measurements should be taken: 1) determine how much soda is placed in a spoon (for this you need to look in a culinary guide or “scoop out” a package of soda of a known mass with this spoon); 2) determine the amount of water - in a small amount of water, the solution will immediately become saturated and part of the soda will not dissolve, in in large numbers water temperature will change by fractions of a degree, making measurements difficult.
Obviously, the amount of heat required to dissolve a substance is proportional to the mass of this substance: Q~m. To record equality, enter the proportionality factor, for example z, which can be called "specific heat of dissolution". Then
Q= zm.
The dissolution of soda is carried out due to the energy released when the vessel with water is cooled. The value of z is found from the following heat balance equation:
mvcv(t 2 -t 1 )+ma cc (t 2 -t 1 ) = zm.
where m v is the mass of water in the calorimeter, m a is the mass of the inner aluminum cup of the calorimeter, m- mass of dissolved soda, ( t 2 -t 1) - lowering the temperature in the calorimeter. The mass of the inner vessel of the calorimeter can be easily found using the rule of moments of force by balancing the vessel and the weight of a known mass using a ruler and string.
Measurements and calculations show that at m= 6 g and m v = 100 G water cools down by 2-2.5 є C, and the value z turns out to be equal to 144-180 kJ/kg.
Task 9. Pot capacity
The task. How can you find the capacity of a pan using a scale and a set of weights?
Prompt. Weigh the empty pot and then the pot of water.
Solution. Let the mass of the empty pan be m 1 , and after filling with water it is m 2. Then the difference m 2 -m 1 gives the mass of water in the volume of the pan. Dividing this difference by the density of water from, find the volume of the pan:
Task 10. How to divide the contents of a glass
The task. There is a cylindrical glass filled to the brim with liquid. How to divide the contents of the glass into two completely equal parts, having one more vessel, but of a different shape and somewhat smaller size?
Prompt. Think about how you can draw a plane that divides the cylinder into two parts of equal volume.
Solution. If through points M And N mentally draw a plane as shown in Figure 1 but, then it will cut the cylinder into two symmetrical and therefore equal in volume figures, in accordance with Figure 8. This implies the solution of the problem.
Gradually tilting the glass, you need to pour out the liquid contained in it until the bottom slightly appears (Figure 1 b). At this point, exactly half of the liquid will remain in the glass.
Electricity
Task 11. Electric "black box"
The "black box" is an opaque closed box that cannot be opened to examine its internal structure. Inside the box are several electrical elements connected to each other in a simple electrical circuit. Usually such elements are: current sources, fixed and variable resistors, capacitors, inductors, semiconductor diodes. Outside the box are several leads.
The main goal of the “black box” task is to “decipher” the “black box” by making the minimum number of electrical measurements using external leads, i.e.:
- - establish which electrical devices are inside the "black box".
- - to establish the scheme of their connection.
- - determine the ratings (resistance values of resistors, capacitor capacitances, etc.)
The task. Three resistors are interconnected and placed in a "black box" with three leads, in accordance with Figure 9. Exactly the same resistors are connected to each other in a different way and placed in a second "black box" with three leads. Determine the resistance of each resistor. Jumpers are not allowed.
Equipment: multimeter.
Measuring the resistance between the leads gave the results:
Drawer #1: R 1-2 = 12Ohm, R 2-3 = 25Ohm, R 1-3 = 37Ohm
Drawer #2: R 1-2 = 5,45Ohm, R 2-3 = 15Ohm, R 1-3 = 20,45Ohm
Possible solution. There are four ways to connect three resistors with three outer leads so that three measurements give different meaning resistance:
1) sequential, 2) mixed, 3) star, 4) delta, in accordance with Figure 10.
Let's show the sequence of search for answers.
A characteristic feature of the first two schemes is that one of the measurements is equal to the sum of the other two, which corresponds to the condition of the problem:
Therefore, in one box there is a serial connection, but then in the other - mixed, since the measurements do not match, although the resistor values are the same.
It is known that the relation always holds
And since R 1-3 left more than R 1-3 on the right, then in the left box (No. 1) there is a serial connection, and in the right (No. 2) - mixed.
The serial connection in the left box includes resistors with ratings of 12 or 25 Ohm. Since neither one nor the other value is observed as part of a mixed connection, therefore, the value of one of the resistors R 1 = 15Ohm.
Other denominations: R 2 = 12Ohm And R 3 = 10Ohm.
Obviously, the same results can be reached with the help of a different chain of reasoning.
We also note that there are 5 more combinations of schemes, each with two "black boxes" out of the four given. The most cumbersome mathematical part of the problem is to "decode" the black box, which is known to contain a triangle.
In conclusion, we note that not everything can go as smoothly as in this example. The values of resistances or other electrical quantities, of course, contain errors. And, for example, the ratio can be fulfilled only approximately.
Task 12. Air temperature in the room
The task. It's snowing outside, but the room is warm. Unfortunately, there is nothing to measure the temperature - there is no thermometer. But on the other hand, there is a battery, a very accurate voltmeter and the same ammeter, as much copper wire as you like, and a detailed physical reference book. Is it possible to use them to find the air temperature in the room?
Prompt. When a metal is heated, its resistance increases linearly.
Solution. We connect the battery in series, turn on the coil of wire and turn on the ammeter so that it shows the voltage on the coil, in accordance with Figure 11. Let's record the readings of the instruments and calculate the resistance of the coil at room temperature:
After that, we will bring snow from the street, immerse a skein in it and, after waiting a bit for the snow to begin to melt, and the wire to its temperature, in the same way we determine the resistance of the wire R 0 at the temperature of melting snow, i.e. at 0 є FROM. Using then the relationship between the resistance of the conductor and its temperature
find the air temperature in the room:
The calculation uses the value of the temperature coefficient of resistance b taken from the handbook. At room temperature for pure copper b= 0,0043 hail - one . If the content of impurities in the copper from which the wire is made is not particularly high, and electrical measuring instruments have an accuracy class of 0.1, then the air temperature can be determined with an error much less than one degree.
Optics
Task 13.
The task. It is required to find the radius of a spherical mirror (or the radius of curvature concave lens) with a stopwatch and a steel ball of known radius. How to do it?
Prompt. The center of a ball rolling on the surface of a mirror makes the same movement as a pendulum.
Solution. You should place the mirror horizontally and lower the ball on it. If the ball is not lowered to the lowest point, it will begin to move along the surface of the mirror. It is easy to guess that if the ball moves without rotation (i.e., slides on the surface of the mirror), then its movement is completely similar to the movement of a pendulum with a suspension length R - r. Then from the pendulum formula
we can find the quantity we are interested in:
Period T determined using a stopwatch, and r known by convention.
Since the friction is usually strong enough for the ball to move on the surface of the mirror with rotation, this solution does not agree well with experiment. Actually
Let's give an example of a research problem for the whole lesson.
Task 14. Oscillation features of a torsion pendulum.
The task. Explore the features of the oscillation of a torsion pendulum and describe the main patterns of its movement.
Equipment: a tripod with a clutch and foot, pieces of copper, steel and nichrome wire approx. 1m and various diameters, for example 0.3, 0.50, 0.65, 1.0 mm, thin light wooden stick 15-20 long cm, plasticine, paper clip, ruler, protractor, stopwatch.
The general view of the torsion pendulum should be in accordance with Figure 12. A clip, bent in a certain way, serves to balance the rod with the weights. The pendulum, taken out of the state of equilibrium, begins to perform rotational-oscillatory motion.
In advance, you need to make pairs of balls of different masses from plasticine. The masses of the balls are proportional to the cube of their diameters, so it is possible to build a series, for example: m 1 = 1, m 2 = 2,5, m 3 = 5,2, m 3 = 6,8, m 4 = 8,3 rel. units
The diameter of the wires can be given to students in advance, or they can be given the opportunity to take these measurements themselves using a caliper or micrometer.
Note. The success of the study largely depends on the correct selection of equipment, especially the diameters of the issued wires. In addition, it is desirable that the suspension of the torsion pendulum be in a taut state during the experiments, for which the masses of the weights must be sufficiently large.
The theme of the study of a torsion pendulum follows from the assumption of the harmonic nature of its oscillations. The general list of experimental observations that can be carried out on this problem and on the proposed equipment is quite large. Here are the most simple and affordable.
- - Does the period of oscillations depend on the amplitude (angle of rotation)?
- - Does the period of oscillation depend on the length of the pendulum suspension?
- - Does the period of oscillation of the pendulum depend on the mass of the weights?
- - Does the period of oscillation of the pendulum depend on the position of the weights on the rod?
- - Does the oscillation period depend on the wire diameter?
Naturally, it is required not only to answer the questions in monosyllables, but also to investigate the nature of the expected dependencies.
Using the technique of analogies, we put forward hypotheses about the oscillations of a torsion pendulum, comparing it with a mathematical pendulum studied by school curriculum. We take as a basis the oscillation period and its dependence on various parameters of the pendulum. We propose the following hypotheses. Oscillation period of a torsion pendulum:
At small angles of rotation does not depend on the amplitude;
- - proportional to the square root of the length of the suspension - T;
- - proportional to the square root of the mass of the load - T;
- - proportional to distance from the center of suspension to the centers of loads - Tr;
- - inversely proportional to the square of the wire diameter - T1/d 2 .
In addition, the oscillation period depends on the suspension material: copper, steel, nichrome. There are also a number of hypotheses here, we suggest checking them yourself.
1. We study the dependence of the period of oscillation of the pendulum on the amplitude (angle of rotation). The measurement results are presented in table 3:
Table 3
The dependence of the period of oscillation of the pendulum on the amplitude
L= 60cm, m = 8,3r, r = 12cm, d= 0,5mm
Output. In the range up to 180, the dependence of the oscillation period of the torsion pendulum on the amplitude is not detected. The scatter of measurement results can be explained by errors in measuring the oscillation period and random causes.
To "open" other dependencies, you need to change only one parameter, leaving all others unchanged. Mathematical processing of results is best done graphically.
2. We study the dependence of the period of oscillation of the pendulum on its length: Т = f(l). At the same time, we do not change m, r, d. The measurement results are presented in table 4:
Table 4
The dependence of the period of oscillation of the pendulum on the length
m = 8,3rel. units, r = 12cm, d= 0,5mm
dependency graph T from l is a curved ascending line similar to a dependency, according to Figure 13 but T 2 =l, in accordance with Figure 13, b.
Output. The period of oscillation of a torsion pendulum is directly proportional to the square root of the length of the suspension. Some scatter of points can be explained by errors in measurements of the period of oscillations and the length of the pendulum
3. We study the dependence of the period of oscillation of the pendulum on the mass of goods: Т=f(m). At the same time, we do not change l, r, d. The measurement results are presented in table 5:
Table 5
The dependence of the period of oscillation of the pendulum on the mass of loads
l = 0,6m, r= 12cm, d= 0,5mm
dependency graph T from m is a curved ascending line similar to a dependency, as shown in Figure 14 but. To verify this, we build a dependency T 2 =f(m), according to figure 14 b.
Output. The period of oscillation of a torsion pendulum is directly proportional to the square root of the mass of the weights. Some scatter of points can be explained by errors in measurements of the period of oscillations and masses of goods, as well as random causes.
4. We study the dependence of the period of oscillation of the pendulum on the position of the weights: Т = f(r). At the same time, we do not change l, m, d. The measurement results are presented in table 6:
Table 6
The dependence of the period of oscillation of the pendulum on the position of the weights
m = 8,3rel.un., l = 0,6m, d = 0,5mm
Output. The period of oscillation of a torsion pendulum is directly proportional to the distance r. Some scatter of points can be explained by measurement errors of the oscillation period and distance r as well as random causes.
We study the dependence of the period of oscillation of the pendulum on the diameter of the wire: T = f(d), in accordance with figure 15 . However, we do not change m, r, l.
The measurement results are presented in table 7.
Table 7
The dependence of the oscillation period of the pendulum on the diameter of the wire
m = 8.3 relative units, r = 12 cm, l = 0.6 m
dependency graph T from d represents a falling curve, in accordance with Figure 16 but. It can be assumed that this is a dependence, where n= 1, 2, 3, etc. To test these assumptions, it is necessary to build graphs, etc. Of all such graphs, the graph is the most linear, in accordance with Figure 16 b.
Output. The period of oscillation of a torsion pendulum is inversely proportional to the square of the suspension wire diameter. Some scatter of points can be explained by measurement errors of the oscillation period and wire diameter d as well as random causes.
The conducted studies allow us to conclude that the oscillation period of a torsion pendulum should be calculated by the formula, where k- coefficient of proportionality, which also depends on the elastic properties of the suspension material - torsion modulus, shear modulus.
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